Electronic equipment belonging to modern telephone and data transmission systems is often mounted in metallic apparatus-cabinets which are effective to screen electromagnetic fields. The intention is, in part, to screen the equipment housed in the cabinet from the effect of external electromagnetic fields, and also to screen the surroundings from undesirable electromagnetic fields generated by the equipment in the cabinet. However, since the transmission lines are able to function as antennas, outside and inside the cabinet, radio-frequency interferences can, nevertheless, be transmitted from the outside to the inside of the cabinet, and vice versa, through these transmission lines. The effect of such interferences diminishes when the transmission lines are connected in series with filters which, while operative to dampen undesirable interferences, will permit the transmission of desired utility signals.
Utility signals occur as so-called symmetrical signals in twin-conductor lines and under certain conditions will remain unaffected, or at least substantially unaffected, by the presence of a filter. This is because the magnetic fields generated in the coils by these signals can be caused to cancel out each other, to a substantial extent. On the other hand, undesirable interference signals, which have been induced in both conductors and which consequently behave as so-called asymmetric signals, are dampened effectively under the same conditions. A relatively low limit frequency for assymetric interference signals can be obtained by appropriate configuration of the filter.
It is known, to this end, to use filters which include coils. One 5 drawback with filters of this design, however, is that they are bulky and heavy, primarily because the coils comprise a conducting wire which is wound around a coil body, and the coils will often include an iron core. Consequently, a filter device which is intended for a plurality of transmission-line pairs and which includes a corresponding number of filters of the aforesaid kind will demand a large amount of space in which to accommodate the coils required, and will also be heavy and difficult to handle.